Sorry! I missed this thread and only just found it!
The large mass of rock in the centre of the quarry was known as Y Talcen Mawr (The Big Needle). It was "badrock", so left and worked around. Estimates vary as to its volume from 125,000 to 200,000 tons. It was destroyed on April, 27. 1895 when 7 tons of explosive were used. Inferences seem to indicate that dynamite was used, in as little as two chambers.
The Prince and Princess of Wales, with their children, were present on the occasion and this was the culmination of a large show of blasting all over the quarry. The journey of the Royal party to the quarry was accompanied by the widespread use of rock cannon in the area. Many of these are still extant.
The view of the quarry from Bryn Ogwen, Braichmelyn, is no longer possible because of the trees that were planted in the 1960's.
Penmorfa has a few pictures and details.
www.llechicymru.info/IPSLRoyalVisitNW.cymraeg has a short account of the visit.